Residual-current circuit breaker and a method for testing the reliability perfromance of a residual-current circuit breaker

ABSTRACT

The aim of the invention is to provide a test circuit ( 16 ) which is independent of the power supply and which enables a circuit breaker ( 2 ) to be tested in a reliable manner. To achieve this, the test circuit ( 16 ) comprises a test coil ( 18 ), which is devoid of electric potential and is wound round a totalizing current transformer ( 4 ). Said test coil is preferably short-circuited using a test switch ( 20 ) and a connectable load (R 2 ). This simulates the occurrence of a residual current. The selection of an appropriate connectable load (R 2 ) allows the sensitivity of the circuit breaker ( 2 ) to be tested in an advantageous manner.

[0001] This application is the national phase under 35 U.S.C. §371 ofPCT International Application No. PCT/EP01/04618 which has anInternational filing date of Apr. 24, 2001, which designated the UnitedStates of America, the entire contents of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

[0002] The invention generally relates to a fault-current circuitbreaker. In particular, it relates to a differential-current circuitbreaker, having a core-balance current transformer and a control windingwound around it. The invention also generally relates to a method forchecking the reliability of such a fault-current circuit breaker.

BACKGROUND OF THE INVENTION

[0003] Fault-current circuit breaker are used in electrical systems, inorder to protect personnel against dangerous body currents. When a faultcurrent occurs, the circuit breaker disconnects the conductors of aconductor network. The circuit breaker is used as an autonomous unit, orelse as an additional module for a switching device. Such an additionalmodule is referred to as a circuit breaker accessory.

[0004] With regard to circuit breakers, a distinction is drawn betweenmain voltage independent, so-called FI switches (fault-current circuitbreakers) and main-voltage-dependent DI circuit breakers(differential-current circuit breakers). Both switch types have acore-balance current transformer, through which the conductors of aconductor network are passed. A control winding is wound around thecore-balance current transformer and is connected to an evaluation unit,via which a release is actuated. When an unacceptable fault currentoccurs in the conductor network, this is detected by the core-balancecurrent transformer with the associated evaluation unit, and the releasedisconnects the conductors of the conductor network via a switchingmechanism. The fault current at which the circuit breaker responds isreferred to as the tripping fault current. The ratio of the trippingfault current to the so-called rated fault current is fixed and isdefined by Standards for the various fault current types. The ratedfault current is a measure of the protection class for which therespective circuit breaker is designed.

[0005] As a rule, FI/DI circuit breakers have a test device, by whichthe reliability of the circuit breaker can be checked. Such a testdevice normally connects two primary conductors to one another via aseries circuit comprising a test resistor and a test winding and via atest contact (pushbutton) which can be operated, forming a test circuit.A fault current is thus produced when the test contact is closed, andthis is detected by the core-balance current transformer together withthe associated evaluation unit. A test device such as this can be found,for example, in the article “Fehlerstrom-Schutzschalter zum Schutz gegengefährliche Körperströme”, [Fault current circuit breaker for protectionagainst dangerous body currents] etz, Volume 110 (1989), Issue 12, pages580-584.

[0006] When two conductors are short-circuited for test purposes, thereis a problem in that, in some circumstances, a test current will flowpermanently via the test circuit for as long as the test contact isoperated. This problem occurs when connection of the test circuit to theconductors of the conductor network is made on the mains voltage supplyside, that is to say upstream of the circuit breaker switchingmechanism, so that current continues to flow in the test circuit evenafter the conductors have been disconnected by the circuit breakerduring the test. Thus, in conventional test devices, an auxiliary switchwhich is coupled to the switching mechanism of the circuit breaker isoften connected in the test circuit, and interrupts the test circuitwhen the circuit breaker trips, in order to ensure that the current flowis reliably interrupted. However, the arrangement of an auxiliary switchon the one hand requires additional measures and on the other hand isnot always possible, for example in the case of FI/DI accessories, sincethere is no switching mechanism for space reasons. If it is impossibleto arrange any auxiliary switches, the test circuit must therefore bedesigned, for example, for permanently flowing test current. The designfor permanent excitation is extremely complex, in particular when thecircuit breaker is designed for high-rated fault currents.

SUMMARY OF THE INVENTION

[0007] An embodiment of the invention is based on an object ofspecifying a fault-current circuit breaker and/or a method for checkingits reliability which allows the circuit breaker to be configured in asimple and functionally reliable manner.

[0008] An object with regard to the circuit breaker can be achievedaccording to an embodiment of the invention by a fault-current circuitbreaker, in particular a differential-current circuit breaker, with acore-balance current transformer and with a control winding wound aroundit. A main-voltage-independent test circuit is preferably provided, witha floating test winding wound around the core-balance currenttransformer.

[0009] In contrast to the known test device, in which an actual faultcurrent is produced by short-circuiting two conductors of the conductornetwork, an embodiment of the invention can be based on the idea of justsimulating a fault current. This avoids the power loss problemsassociated with a test current flowing for an undefined time. Thecritical element for simulation of a fault current is the floating testwinding, that is to say a coil which is wound around the core-balancecurrent transformer and has no connection for the conductors of theconductor network.

[0010] The simulation is based on the principle that the magnetizationof the core-balance current transformer is varied by the inductionprinciple as a function of the short-circuit resistance of the testwinding. This effect also occurs in the case of a fault current, in thiscase, as the magnetic fields of the conductors which are passed throughthe core-balance current transformer no longer cancel one another out.The change to the magnetization of the core-balance current transformeris—as is normal for all-current-sensitive DI circuit breakers—detectedby the control winding, which is stimulated by an AC voltage, and by theassociated evaluation unit. The evaluation process is in this casecarried out essentially on the basis of the permeability, which can bemeasured or determined via the control winding, of the core-balancecurrent transformer, and is dependent on the change in magnetization ofthe core-balance current transformer.

[0011] In one preferred embodiment, the test circuit has a test switchvia which the test winding can be short-circuited. This allows the testcircuit to be designed with particularly simple circuitry. Theexpression test switch also means, in particular, a test button.

[0012] The test circuit is in this case preferably designed without aseparate voltage supply. The test is thus carried out in a floatingmanner, in the sense that there is no specific voltage source. In fact,it is sufficient to use for the test the voltage which is induced in thetest winding by the alternate magnetization of the core-balance currenttransformer by the control winding.

[0013] In one particularly expedient refinement, the test circuit has aburden which can be connected and which is used to influence thepermeability which can be measured via the control winding. The choiceof the burden in this case advantageously makes it possible to selectthe permeability which can be measured, thus simulating a specificfault-current level. The arrangement of the burden thus makes itpossible to check the sensitivity of the circuit breaker.

[0014] The burden is in this case preferably formed by a resistorconnected in series with the test switch. If the test circuit is in theform of a short-circuiting circuit, the short-circuit is thus producedvia the resistor, which is then arranged in parallel with the testwindings.

[0015] In one particularly expedient refinement, the burden which can beconnected is designed such that, when the test contact is closed atripping criterion which is predetermined for the circuit breaker issatisfied, or is more than satisfied by a defined amount.

[0016] As already mentioned, the burden offers the capability to checkthe sensitivity of the circuit breaker. If the burden which can beconnected is designed such that the tripping criterion which ispredetermined for the circuit breaker by Standards is satisfied exactly,it is possible to identify rises in the tripping fault current above thepermissible limit value. If the burden is chosen such that the trippingcriterion is more than satisfied by a specific amount, it is possible toensure correct tripping even in the event of poor component tolerances.The tripping criterion would be exceeded by several times in the case ofa pure short-circuit winding with a resistance of zero ohms. A test suchas this thus relates to a pure functional test of mechanicaldisconnection of the conductors. The choice of the burden fordetermining the tripping criterion provided for the circuit breaker inthis case depends on the design of the circuit breaker, for example, onthe number of windings for the control winding.

[0017] With the previous method of short-circuiting two conductors, itwas impossible to check the sensitivity, or this could be carried outonly to a highly restricted extent, since the rated voltage range isnormally wide. This is because, with a conventional test device,insensitivity of the circuit breaker remains undetected up to a certainlevel. This is because there is a risk of the circuit breaker nottripping at the specified tripping fault current, but only at a multipleof it, as a result of a functional defect. A functional check based onthe conventional method would not detect this functional defect, sincethe tripping fault current would be considerably exceeded byshort-circuiting of the conductors. The test method provided by thefloating test winding thus allows considerably better results to beobtained with respect to the serviceability of the circuit breaker, thana conventional test device. In particular, there is no risk of afunctional defect remaining undetected and, in the worst case, injuringsomeone if a fault current were to occur.

[0018] In one advantageous embodiment, the burden which can be connectedis variable, in particular in order to make it possible to checkdifferent sensitivities for circuit breakers whose rated fault currentis adjustable. The variability of the burden is achieved, for example,by using a variable potentiometer in the test circuit, or else by usingdifferent resistors, for example in conjunction with a multi-stagerotary switch in the test circuit.

[0019] According to one particularly expedient refinement, the testcircuit has a continuously acting burden, which influences thepermeability which can be measured via the control winding.

[0020] In conventional circuit breakers, such a permanent burden isoften arranged in parallel with the control/or secondary winding, inorder to define the tripping response of the circuit breaker. However,this has the disadvantage that a current flows via the burden, which isarranged in parallel to the control winding, and this makes it harder toevaluate the voltage drop across the measurement resistor, which isconnected in series with the control winding, as a measure of themeasured permeability. With regard to the tripping response, thearrangement of such a permanently acting burden in the test circuit hasthe same effect as the arrangement in parallel to the control winding,but offers the major advantage that it considerably simplifies theevaluation of the voltage drop across the measurement resistor, which isconnected in series with the control winding.

[0021] The permanently acting burden is preferably available, so that itis possible to set the tripping fault current and/or the rated faultcurrent. In conjunction with the capability to check different trippingfault currents by means of the variable burden which can be connected,it is thus possible to ensure correct tripping even with poor componenttolerances.

[0022] The test winding is preferably wound symmetrically around thecore-balance current transformer. The symmetrical or uniform windingaround the core-balance current transformer in this case ensures thatthe resultant inductive effect of the inhomogeneous magnetic fieldscaused by the load currents is cancelled out overall, and that nodisturbance voltages are thus induced in the winding. This isparticularly necessary when there is a permanently acting burden in thetest circuit since this results in the core-balance current transformerbeing burdened independently of the field distribution. The uniformwinding results in non-uniform field distributions in the transformercore, for example caused by the dipole field of the load current, beingaveraged out over the circumference of the core-balance currenttransformer.

[0023] In one expedient refinement, the test circuit has an additionalswitch or push button in order to trip the circuit breaker remotely, viawhich the test winding can be short-circuited.

[0024] In particular, the floating configuration of the test circuit isadvantageous with regard to the safety requirements for such remotetripping. At the same time, the test winding is expediently sufficientlywell isolated from the control winding, which is electrically connectedto the conductor network, in order to satisfy the safety requirements,which demand safe conductive isolation between a remote tripping circuitand the conductor network.

[0025] In one preferred alternative, a further winding is providedaround the core-balance current transformer, in addition to the testwinding, for remote tripping. This further winding can preferablylikewise be short-circuited. If the circuit for remote tripping has nopermanently acting burden, a small number of turns in the furtherwinding are sufficient to ensure operation of the remote tripping. Inthis case, the winding need not be designed to be symmetrical. This hasthe advantage that it simplifies the isolation of the control winding.

[0026] According to one embodiment of the invention, the reliability ofa fault-current circuit breaker can be simulated by a test circuithaving a test winding wound around the core-balance current transformer.

[0027] The advantages and preferred embodiments mentioned with regard tothe circuit breaker can be transferred in the same sense to the method.Particularly expedient refinements of the method are specified in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Exemplary embodiments of the invention will be explained in moredetail in the following text with reference to the drawings in which, ineach case illustrated schematically:

[0029]FIG. 1 shows a circuit diagram of a circuit breaker with a testcircuit,

[0030]FIG. 2 shows a circuit diagram of a circuit breaker with amodified test circuit and a separate remote tripping circuit,

[0031]FIG. 3 shows a B-H diagram with different magnetization curves,

[0032]FIG. 4 shows a detail from the circuit arrangement of a circuitbreaker with a burden arranged in parallel with the control winding ofthe circuit breaker, and

[0033]FIG. 5 shows a detail of a circuit arrangement of a test switchwith a permanent burden arranged in parallel with the test winding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] As shown in FIG. 1, a fault-current circuit breaker 2 has acore-balance current transformer 4, a control winding 6 wound around it,and a functional unit 8. The latter not only has actuation andevaluation electronics for the control winding 6, but also a trippingmechanism. The conductors L1, L2, L3 and the neutral conductor N of aconductor network are passed through the core-balance currenttransformer 4. Each conductor L1-L3, N has an associated interrupterswitch 10, via which the conductors L1-L3, N are disconnected by meansof a switching mechanism 12, which is represented by dashed lines, whenan unacceptable fault current occurs.

[0035] Supply lines 14 lead from the individual conductors L1-L3, N tothe functional unit 8, in order to provide a power supply for theelectronics integrated in it. The circuit breaker 2 as shown in FIG. 1is thus, by definition, in the form of a main-dependent DI circuitbreaker.

[0036] In addition to the already described elements, which every DIcircuit breaker 2 has, the circuit breaker as shown in FIG. 1 has, as anessential new feature, a test circuit 16 with a test winding 18 woundaround the core-balance current transformer 4. A permanently actingburden R1 in the form of a resistor is provided in parallel with thetest winding 18. The test circuit 16 has a test switch 20, via which thetest winding 18 can be short-circuited via a further burden R2 which canbe connected. The latter is likewise in the form of a resistor, which isarranged in series with the test switch 20. In the exemplary embodimentshown in FIG. 1, the test winding 18 is at the same time part of aremote tripping circuit 22, which is connected via a remote trippingline 24 to the test circuit 16 and has a switch 26 which is arranged inparallel with the test switch 20.

[0037] For safety reasons, and in order to ensure that the circuitbreaker is resistant to surge currents, a voltage-limiting element maybe provided in parallel with the control winding and/or in parallel withthe test winding.

[0038] The exemplary embodiment of a circuit breaker 2 shown in FIG. 2differs from that shown in FIG. 1 in that the remote tripping circuit 22is in the form of a separate remote tripping circuit 22 with its ownwinding 30, and in that the permanently acting burden R1 and the burdenR2 which can be connected are configured as variable resistors, in theform of a double potentiometer. The remote tripping circuit 22 can inthis case be short-circuited via the switch 26 and via a resistor whichacts as a burden R3, in order to cause the circuit breaker R2 to trip.

[0039] The method of operation of the test circuit 16 for checking thereliability of the circuit breaker 2 will be explained in conjunctionwith FIG. 3 in the following text. FIG. 3 shows a B-H diagram,illustrating a number of magnetization curves I-IV. The magneticinduction B is plotted on the ordinate, against the magnetic fieldstrength H on the abscissa. The individual magnetization curves I-IVhave different gradients, with the magnetization curve I bendingconsiderably into a saturation region above a specific magnetic fieldstrength H. The gradient of the individual magnetization curves I-IVcorresponds to the permeability μ, as detected by the control winding 6,of the core-balance current transformer 4. The permeability μ measuredby the control winding 6 is governed by the actual permeability of thecore-balance current transformer 4 and by superimposed effects. Onesuperimposed effect, by way of example, is the occurrence of a faultcurrent in the conductor network, or else a burden. Both effects cause achange to the profile of the magnetization curve and are detected by thecontrol winding with the associated evaluation unit. The permeability μ,that is to say the gradient of the magnetization curve, is generally setby the permanent burden R1. In this case, the gradient of themagnetization curve becomes ever flatter, as the resistance of theburden R1 is decreased. The tripping response of the circuit breaker 2is also governed by the permanent burden R1.

[0040] An AC voltage is applied to the control winding 6, so that thecore-balance current transformer 4 is magnetized alternately. Themagnetization curve is in this case evaluated at an operating point Hafor a defined magnetic field strength H. This makes use of the fact thatthe coil resistance of the control winding 6 is high when thepermeability μ is high, and is correspondingly reduced when thepermeability is less. The voltage drop across the control winding 6 isevaluated via a measurement resistor 28 (in this context, see FIG. 4 andFIG. 5).

[0041] The test winding 18 is terminated via the permanently actingburden R1. The alternate magnetization of the core-balance currenttransformer 4 via the control winding 6 results in a voltage beinginduced in the test winding 18, so that a current flows in the testcircuit 16 which results in the test winding 16 producing a magneticfield which counteracts the magnetization of the core-balance currenttransformer 4 caused by the control winding 6. The permeability μmeasured by the control winding 6 is thus less than the actualpermeability of the core-balance current transformer 4.

[0042] When the test switch 20 is operated, the further burden R2 isconnected, so that the measurable permeability μ is changed once again.If the resistance of the further burden R2 is in this case reduced, thisresults in a greater change in the permeability μ. The burden R2 whichcan be connected is now preferably chosen such that the change caused inthis way to the measurable permeability μ corresponds to the situationwhen a fault current occurs, for example a tripping fault current, inresponse to which the circuit breaker 2 disconnects the conductorsL1-L3, N. The connection of the burden R2 therefore simulates theoccurrence of a fault current.

[0043] The major advantage of this test method is that the choice of asuitable resistance for the burden R2 makes it possible to simulatetripping fault currents of different magnitudes so that it is possibleto check the sensitivity of the circuit breaker 2. Furthermore, the testcircuit 16 does not require a separate voltage supply. This is becauseits principle of operation results in a voltage being induced via thetest winding 18 in the test circuit 16.

[0044] A further advantage of the test circuit 16 is that it can at thesame time be used for remote tripping. This can be done just byconnecting appropriate remote tripping lines 24 to the test circuit 16.In particular, the floating configuration of the test circuit 16 isadvantageous with regard to the safety requirements for such remotetripping. If the test circuit 16 is at the same time used for remotetripping, then it is necessary to ensure that the test winding 18 issufficiently well isolated from the control winding 6, which is normallyat the same potential as the main circuit.

[0045] If the permanent burden R1 is arranged in the test circuit 16 asshown in FIG. 1, the test winding 18 is preferably arrangedsymmetrically and uniformly around the core-balance current transformer4. This results in the core-balance current transformer 4 being burdenedindependently of a field distribution, in order to avoid errors in theevaluation resulting from inhomogeneities in the magnetic fields. Suchinhomogeneities are caused by an asymmetric arrangement of theconductors L1-L3, N in the core-balance current transformer 4 so that,even when no fault current is flowing, local magnetic fields occur whichlead to local magnetization in the core-balance current transformer 4.The induction effects of these local magnetizations cancel one anotherout overall only when a winding is distributed homogeneously on thetransformer core.

[0046] The advantageous arrangement of the permanent burden R1 inparallel with the test winding 18, instead of the arrangement inparallel with the control winding 6, will be explained with reference toFIG. 4 and FIG. 5.

[0047]FIG. 4 in this case shows the conventional arrangement of theburden R1 in parallel with the control winding 6, and FIG. 5 shows thenew arrangement of the permanent burden R1 within the test circuit 16.An AC voltage is applied to the control winding 6 via a voltagegenerator 32. The already mentioned measurement resistor 28, which isused to detect the voltage drop across the control winding 6 as ameasure of the measurable permeability, is in each case arranged inseries with the test winding 6. An evaluation circuit 34 is provided inparallel with the measurement resistor 28. In the arrangement shown inFIG. 4, a current element I1 flows via the control winding 6, and acurrent element I2 flows via the permanent burden R1. The voltage drop Uacross the measurement resistor 28 is governed by the two currentelements I1, I2. In contrast to this, and according to the exemplaryembodiment shown in FIG. 5, all the current I1′ flows via the controlwinding 6. This simplifies the evaluation of the voltage drop U acrossthe measurement resistor 28.

[0048] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A fault-current circuit breaker (2), in particular a differentialcurrent circuit breaker, having a core-balance current transformer (4)and having a control winding (6) wound around it, characterized in thata mains voltage independent test circuit (16) is provided, having afloating test winding (18) which is wound around the core-balancecurrent transformer.
 2. The circuit breaker (2) as claimed in claim 1,characterized in that the test circuit (16) has a test switch or pushbutton (20), via which the test winding (18) can be short-circuited. 3.The circuit breaker (2) as claimed in claim 1 or 2, characterized inthat the test circuit (16) is designed without a separate voltagesupply.
 4. The circuit breaker (2) as claimed in one of the precedingclaims, characterized in that the test circuit (16) has a burden (R2)which can be connected and influences the permeability (μ) which can bemeasured via the control winding (6).
 5. The circuit breaker (2) asclaimed in claim 4, characterized in that the burden (R2) which can beconnected is formed by a resistor connected in series with the testswitch (20).
 6. The circuit breaker (2) as claimed in claim 4 or 5,characterized in that the burden (R2) which can be connected is designedsuch that a tripping criterion which is predetermined for the circuitbreaker (2) is satisfied or is more than satisfied by a defined amount.7. The circuit breaker (2) as claimed in one of claims 4 to 6,characterized in that the burden (R2) which can be connected isvariable.
 8. The circuit breaker (2) as claimed in one of the precedingclaims, characterized in that the test circuit (16) has a continuouslyacting burden (R1) which influences the permeability (μ) which can bemeasured via the control winding (6).
 9. The circuit breaker (2) asclaimed in claim 8, characterized in that the continuously acting burden(R1) is variable.
 10. The circuit breaker (2) as claimed in one of thepreceding claims, characterized in that the test winding (16) is woundsymmetrically around the core-balance current transformer (4).
 11. Thecircuit breaker (2) as claimed in one of the preceding claims,characterized in that the test circuit (16) has a switch (26), via whichthe test winding (18) can be short-circuited, for remote tripping. 12.The circuit breaker (2) as claimed in one of claims 1 to 10,characterized in that a further winding (30) is provided around thecore-balance current transformer (4), for remote tripping.
 13. A methodfor checking the serviceability of a fault-current circuit breaker (2),in particular a differential-current circuit breaker, which has acore-balance current transformer (4) with a control winding (6) woundaround it, characterized in that the occurrence of a fault current issimulated by a mains voltage independent test circuit (16) with afloating test winding (18) which is wound around the core-balancecurrent transformer (4).
 14. The method as claimed in claim 13,characterized in that the test winding (18) is short-circuited via aburden (R2) which can be connected, so that the permeability (μ) whichcan be measured via the control winding (6) assumes a defined value. 15.The method as claimed in claim 13 or 14, characterized in that avariable burden (R2) is used.